The MRI apparatus is an apparatus that measures an NMR signal generated by the object, especially, the spins of nuclei which form human tissue, and images the shapes or functions of the head, abdomen, limbs, and the like in a two-dimensional manner or in a three-dimensional manner. In the imaging, NMR signals are subjected to different phase encoding by the gradient magnetic field and subjected to frequency encoding, and are measured as time-series data. The measured NMR signals are reconstructed as an image by a two-dimensional or three-dimensional Fourier transform.
As one of the imaging methods, there is fast spin eo (FSE). The FSE is an imaging method for reducing the imaging time by acquiring a plurality of echoes (echo trains) by applying a plurality of refocus RF pulses (180° pulses) for one excitation RF pulse (90° pulse). Such an imaging method is referred to as multi-echo imaging. In the echo train, transverse magnetization is attenuated in transverse relaxation time T2 (T2 attenuation). Accordingly, if the echo train is too long, the signal strength may be insufficient in the echo near the end.
As a method for solving this, there is a method called variable refocus flip angle (VRFA) to change the flip angle (FA) of a refocus RF pulse. In the VRFA, the signal strength is prolonged by making the components of the longitudinal magnetization be included in the echo signal by changing the FA appropriately. In addition, the longitudinal magnetization is recovered in the longitudinal relaxation time T1, but is attenuated in the T1 when components contributing to the generation of the echo signal in the echo train are taken into consideration. In this specification, this is referred to as T1 attenuation. In many tissues, T1 is sufficiently longer than T2. Therefore, in the VRFA, attenuation in the echo train can be delayed by making the components of the longitudinal magnetization be included.
Since the relaxation times T1 and T2 are tissue-specific characteristics, the relaxation times T1 and T2 are determined by the tissue. There are T1-weighted (T1W) imaging for imaging a difference (T1 contrast) in the relaxation time T1 between two different tissues using this and T2-weighted (T2W) imaging for imaging a difference (T2 contrast) in the relaxation time T2 between two different tissues using this. When acquiring the T1-weighted image, the maximization of the T1 contrast and the reduction of the T2 contrast are important. In addition, when acquiring the T2-weighted image, maximization of the T2 contrast and reduction of the T1 contrast are important. Generally, in the SE sequence including the FSE, the influence due to the difference in the relaxation time T1 (influence due to T1 attenuation; T1 contrast) can be reduced by setting the TR (repetition period) to be long, and the influence due to the difference in the relaxation time T2 (influence due to T2 attenuation; T2 contrast) can be reduced by setting the TE (echo time) to be short. The TR does not affect the T2 contrast, and the TE does not affect the T1 contrast.
However, in the VRFA, since the components of the longitudinal magnetization are also included in the echo signal, a change in the signal strength within the echo train is based on both the T1 attenuation and the T2 attenuation. That is, both the influence of the T1 attenuation and the influence of the T2 attenuation are mixed. Therefore, in the VRFA, the TE affects both of the T1 contrast and the T2 contrast. The VRFA is a technique for making the echo train as long as possible. However, if the echo train is too long, T2 attenuation in the echo train affects the T2 contrast. Accordingly, the length of the echo train is also limited in this respect.
In such a VRFA, in order to maximize the T1 contrast when acquiring the T1-weighted image, there is a method of reversing the magnetization in the last part of the echo train (for example, refer to PTL 1). In this method, since the signal difference is maximized at the head of the echo train, reversal of the contrast due to the T2 attenuation in the echo train is unlikely to occur. Accordingly, the T1 contrast is maintained over the entire echo train.